Force and laws of motion Worksheet-9

A boy of mass 50 kg running at 5 m/s jumps on to a 20 kg trolley traveling in the same direction at 1.5 m/s. what is their common velocity?

A man of mass 50 kg jumps out of a rowing boat of mass 300 kg on to the bank, with a horizontal velocity of 3 m/s. with what velocity does the boat begin to move backwards?

A tanker of mass 500 kg moving at 4 m/s collides with another tanker of mass 1500 kg moving in the same direction at 2 m/s. What is their common velocity just after the collision if they move off together ?

A ball X of mass 1 kg travelling at 2 m/s has a head-on collision with an identical ball Y at rest. X stops and Y moves off. Calculate the velocity of Y after the collision.

A heavy car A of mass 2000 kg travelling at 10 m/s has a head-on collision with a sports car B of mass 500 kg. If both cars stop dead on colliding, what was the velocity of car B ?

A man wearing a bullet-proof vest stands still on roller skates. The total mass is 80 kg. A bullet of mass 20 grams is fired at 400 m/s. It is stopped by the vest and falls to the ground. What is then the velocity of the man ?

(a) Define momentum of a body. On what factors does the momentum of a body depend?

(b) Calculate the change in momentum of a body weighing 5 kg when its velocity dedreases from 20 m/s to 0.20 m/s.

(a) Define the term 'force'.

(b) State the various effects of force.

Give one example each where :

(a) a force moves a stationary body.

(b) a force stops a moving body.

(c) a force changes the speed of a moving body.

(d) a force changes the direction of a moving body.

(e) a force changes the shape (and size) of a body.

(a) What do you understand by the terms "balanced forces" and "unbalanced forces" ? Explain with examples.

(b) What type of forces - balanced or unbalanced - act on a rubber ball when we press it between our hands ? What effect is produced in the ball ?

What happens to the passengers travelling in a bus when the bus takes a sharp turn ? Give reasons for your answer.

State and explain Newton's second law of motion.

A 1000 kg vehicle moving with a speed of 20 m/s is brought to rest in a distance of 50 metres :

(i) Find the acceleration.

(ii) Calculate the unbalanced force acting on the vehicle.

Explain why, a cricket player moves his hands backwards while catching a fast cricket ball.

State Newton's third law of motion and give two examples to illustrate the law.

State the law of conservation of momentum.

If a balloon filled with air and its mouth untied, is released with its mouth in the downward direction, it moves upwards. Why?

Answer:

4 m/s

0.5 m/s

2.5 m/s

2 m/s

40 m/s

0.1 m/s

(a) The momentum of a body is defined as the product of its mass and velocity.

Thus, momentum = mass × velocity

or, where p = m × v

m = mass of the body

and v = velocity (or speed) of the body

(b) 99 kg.m/s decrease

A push or pull on a body is called force.

Effects of force :

(a) a force moves a stationary body.

(b) a force stops a moving body.

(c) a force changes the speed of a moving body.

(d) a force changes the direction of a moving body.

(e) a force changes the shape (and size) of a body.

–

Balanced Force: If the resultant of all the force acting on a body is zero, the forces are called balanced force.

Though balanced force cannot produce motion in a stationary body or stop a moving body, they can, however, change the shape of the body.

Example : When we press a balloon or a soft rubber ball between our two hands then a balanced force works and shape of balloon or rubber ball changes.

Unbalanced Force: If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces.

Unbalanced forces can move a stationary body or they can stop a moving body.

Example : To move a stationary toy car we have to push it with a force greater than the opposite force.

When the bus takes a sharp turn the passengers tend to fall sideways because of intertia. The tendency of our body is to move in a straight line but when there comes a turn the body resists the change and falls sideways.

Newton’s Second Law of Motion: The rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction in which the force acts. Newton’s second law of motion can be expressed as:

(b) (i) –4 m/s2 (ii) – 4000 N

A cricket player moves his hands backward while catching a fast cricket ball to prevent injury to his hands. A fast moving ball has a very large momentum because of heavy mass and great speed of cricket ball. So to stop the cricket ball, its momentum has to be reduced to 0. When a player moves his hands back the time taken to reduce the momentum of ball to zero is increased and thus the rate of change of momentum of ball is decreased and only a small force is exerted on the hands of player and this way he doesn’t get hurt.

Newton’s Third Law of Motion: Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the first body.

The force exerted by the first body on the second body is known as “action” and the force exerted by the second body on the first body is known as “reaction” it should be noted that “action” and “reaction” are just forces.

To every action there is an equal and opposite reaction.

Example : While swimming the swimmer pushes the water backwards and the water pushes the swimmer forwards and this way the swimmer moves forward.

Momentum is never created or destroyed

When two (or more) bodies act upon one another, their total momentum remains constant (or conserved) provided no external forces are acting. The law of conservation of momentum means that whenever one body gains momentums, then some other body must lose an equal amount of momentum. The law of conservation of momentum is also known as the principle of conservation of momentum.